Pneumonia virus of mice (PVM) infects mice, rats, hamsters and guinea pigs. Because of the broad spectrum of rodent hosts, PVM has the potential to influence many experimental results, notably, those studies involving the cell dynamics of pulmonary parenchyma. The objective of this project is to study the natural history and transmission of PVM in euthymic and athymic mice. Little is known about the pathology, patent period, epidemiology, and target cell population of this virus. We have observed, and reported for the first time, fatal wasting disease in athymic nu/nu mice naturally infected with PVM. Immunofluorescence studies of lung material from index cases indicate that the target cell in late stage natural infection in the nu/nu mouse is located in the pulmonary parenchyma rather than the lung airways. In addition we have used the electron microscope to confirm the presence, location and structure of the virus in infected mice. Studies are underway to produce a supply of stock PVM virus (TC virus; lung suspension virus from nu/nu mice) for future studies. The stock virus will be assayed using hemadsorption, Infectious Dose 50 and Lethal Dose 50 methods to determine virus concentration. Studies will be undertaken soon to determine means and duration of virus transmission from experimentally infected CD-1 and nu/nu mice to susceptible cage mates (added at specified intervals post infection) and to other mice housed in nearby cages with or without filter bonnets.